There are many occasions when the owners of small animals desire to retain the animal within an area or to exclude the animal from a particular area. Devices of this type are available on the market and are often called an "electronic fence". In the case of pets, which are to be included within or excluded from a controlled area, it is desirable to have a simple system that operates effectively free of normal electrical interferences found in homes and the like. It is also desirable to have a system that is small, lightweight and operates at low power to give good battery life. Inclusionary areas may be a yard, for example. Exclusionary areas may be rooms, beds, tables, chairs, sofas and the like.
Many systems of this general type for either including or excluding animals have been disclosed by the prior art. For example, Peck, (U.S. Pat. No. 3,753,421) discloses a signal emitting wire antenna that delineates an area with respect to which the animal could be included or excluded. The animal wears a receiver and physical effect producer on a collar. The signal emitted by the antenna wire is a modulated alternating current in the sub-broadcast band, i.e., below 560 kHz. The strength of the signal field at the receiver depends on the proximity of the receiver to the antenna. A mild electrical shock (a physical effect) is generated to control the animal and is proportionate to the field strength. Unfortunately the Peck system is sensitive to the orientation of the receiver antenna relative to the wire and may lead to loss of signal. The animal can learn to orient the collar so as to avoid the shock while crossing the wire antenna. Also, the Peck system, especially the transmitter, requires relatively high power to operate and is not amendable to sustained battery operation.
This orientation sensitivity is alleviated to some extent by Giunta (U.S. Pat. No 4,967,695) who teaches a use of multiple antennas and receivers and the selection of the largest magnitude signal received. Unfortunately such circuitry adds to the complexity of the system and increases its power consumption considerably. The transmitter also requires relatively high power to operate.
An alternate system is disclosed by Weinstein (U.S. Pat. No. 5,067,441) who uses multiple antennas of the magnetic loop (ferrite core) variety. The transmitting loop is a central antenna which produces a radiation lobe characterizing the area in which the animal is to be constrained as contrasted to the wire perimeter type loop of the prior art. Weinstein measures the vector sum of the signals from multiple receivers to derive the magnitude of the field strength vector. Such a system is rather complex as it computes the RMS value of the audio beat frequency and then squares the value before adding the component signals. This system also is power consumptive.
None of these prior art systems use FM. These prior art systems appear to have used amplitude modulation (AM) because of its perceived simplicity and minimal circuitry associated with the low power needed for the collar batteries. One of the conventional advantages of FM is the static-free reception. While that is true when the signal is above some threshold, just below that threshold the received signal is very noisy. In this application it is expected that the animal will challenge the zone of the fence or that the excess noise might trigger inappropriate alarms. This would have unfortunate consequences for the uniform training of the animal. These problems make the advantages of FM to an animal confinement and control system not straightforward and apparently caused dismissal of the concept before the salient advantages were discovered.
Most of these prior art systems require substantial power for the transmitter and must be attached to a conventional power outlet in the home. Such power requirement renders the transmitter stationary, since it is attached to a power cord, and involves potentially lethal voltages which can be dangerous to the animal as well as the human operator. Another problem with such electronic fences is that they are not readily attachable to moveable objects such as beds, tables, chairs, sofas, or other animals for that matter. Attachment of the electronic fence to an animal would permit that animal to be protected from other animals in the same household.